It is becoming increasingly likely that our society will need to remove large quantities of CO2 from the atmosphere in order to prevent climate change. The question then becomes: how are we going to store all of this CO2 that we remove from the atmosphere? The main approaches for carbon sequestration today fall into two buckets: geologic and biologic. Geologic sequestration involves injecting would-be CO2 emissions deep underground into rock formations that trap this CO2 permanently (well, permanently as far as humans are concerned).
Above: Geologic sequestration (USGS)
Biological sequestration techniques work by increasing carbon stocks in terrestrial ecosystems (e.g. forests, soils, grasslands, etc.).
Above: biologic sequestration sources (USGS)
Each of these approaches has their pros and cons, which I'll get to shortly. But the good news is that both sequestration techniques have massive potential for locking carbon away. The figure below from the IPCC shows the magnitude of the planet's carbon sinks in billion tonnes of carbon. Human emissions are a small blip compared to the total sequestration potential of the planet's biologic and geologic stocks of carbon:
On to the pros and cons of geologic and biologic sequestration:
Pros: Today, geologic sequestration is more scientifically proven in its ability to sequester carbon permanently. Decades of enhanced oil recovery (EOR) and carbon sequestration pilot studies have shown that long-term geologic storage is technically feasible, and experts are increasingly confident in its efficacy and permanence.
Cons: The major problem with geologic sequestration right now is economics: unless geologic sequestration is coupled with EOR, there aren't many sources of revenue associated with the process (especially when carbon prices remain so low and uncertain).* Combined with the fact that drilling and operating large-scale storage wells is fairly capital intensive, the business case for geologic sequestration remains difficult to make.
*There are a number of companies working on "geologically" sequestering CO2 in useful products (such as cement and plastic), but overall CO2 utilization for sequestration only presents a small fraction of the sequestration potential as do other geologic sequestration techniques.
Pros: Unlike geologic sequestration, there are a handful of promising business cases associated with biologic sequestration today. Taking carbon out of the atmosphere is what our plants and soils have been doing for the past few billion years -- and it is relatively inexpensive to change our agriculture and forestry practices to leverage these natural systems to attempt to sequester increased amounts of carbon. Holistic planned grazing of cattle and ecosystem restoration projects provide examples of biologic sequestration approaches.
Given the more promising economics of biologic sequestration approaches, a number of government efforts designed to encourage such biologic sequestration have emerged: the US DOT, the USDA, and numerous other US Federal agencies all have biologic sequestration programs, as does the World Bank through its BioCarbon Fund. Biologic sequestration techniques are gaining increased attention in the climate change dialogue as well, as shown by the videos at the end of this post.
Cons: The main problem with biologic sequestration is measurement and verification: open questions remain around a) how much carbon is truly sequestered through these techniques, and b) how permanent the carbon sequestration is. Many proponents of geologic sequestration rightly worry about what will happen if ecosystems managed for carbon sequestration revert back to conventional practices, or if natural disasters (like wildfires, hurricanes, etc.) will cause carbon carefully sequestered in these ecosystems to escape in massive quantities, thus undoing previous sequestration efforts. But rather than throw the proverbial baby out with the bath water, it is important to understand the true extent of carbon sequestered in terrestrial ecosystems, in order to discount this storage appropriately in comparison to geologic storage. Even if there are large risks around the permanence of biologic sequestration, such biologic approaches might prove critical for buying time to develop more cost-effective geologic sequestration techniques.
The bottom line: In the end, both biologic and geologic sequestration are worth pursuing for sequestering carbon, but more research needs to be done about a) how to improve the business case for geologic sequestration, and b) how to appropriately measure and account for biologic sequestration.